Acoustic emission is the term given to stress waves created in materials by the sudden release of energy resulting from irreversible processes such as crack growth, plastic deformation, and phase transformations in solid materials. Acoustic emission (AE) techniques have been used in the field for over 25 years for the nondestructive (NDE) testing of structures, including metal and composite pressure vessels and piping. It has also found wide application in the testing of composite man-lift booms. Currently, AE techniques are used primarily for locating cracks and potential problem areas in metal structures for pressure boundary applications, while other types of nondestructive techniques are necessary to provide acceptance or rejection criteria. The AE technology has not achieved the same level of acceptance as other nondestructive techniques for the field testing of structures such as bridges and other components of infrastructures for two primary reasons: (1) the difficulty in separating valid signals from those caused by extraneous noise, and (2) the inability of the AE techniques to determine the size of the crack or flaw.
When a crack grows in a metallic material under stress due to fatigue, stress corrosion cracking, or hydrogen embrittlement cracking, a small amount of strain energy is released in the form of a stress wave that propagates from the source of the crack at the velocity of sound in the material. The increment of time in which the strain release occurs is on the order of a microsecond or less. Thus the frequency content of the stress wave is very broad band, ranging from a few kilocycles to over 1megacycle in frequency. In solids, two types of stress waves can exist in the bulk of the material: an extension wave, where the particle motion is parallel to the direction of propagation, and a shear wave, where the particle motion is perpendicular to the direction of propagation.
The energy of the stress wave from the crack usually consists of both an extension wave and a shear wave. Most structures constructed from metal are plate-like in nature, examples of which include pressure vessels, bridges, and aircraft. Therefore, the stress wave shortly after initiation will strike a boundary. If it strikes the boundary at an angle, Snells Law prevails and mode conversion of an extensional wave to a shear wave can occur, while a shear wave can mode-convert to an extension wave. The behavior of the stress wave can become very complicated by the time it has traveled a distance of several plate thicknesses away from the crack. In this situation, the propagating waves will be governed by Lamb's homogeneous equation, the solution to which are known as Lamb waves. In the limit where the wavelength is much larger than the plate thickness, a simpler set of governing equations derived from classical plate theory can be used to model the motion. Under classical plate theory, the waves are called plate waves, and there are two modes of propagation, namely, the extensional mode and the flexural mode. Both have in-plane IP motion and out-of-plane (OOP) motion. The OOP motion is the greatest for initial displacements of a source perpendicular to the plane of the plate, and the (IP) motion is greatest for initial displacements of a source parallel to the plane of the plate. For example, the sudden propagation of a crack will create primarily an IP wave, because the crack normally grows in a direction perpendicular to the plane of the plate, while impacts on the surface of the plate will create primarily OOP sources, since the initial source function creates a bending or flexural wave.
The stress waves (acoustic emission (AE) events) in present practice are detected by a piezoelectric transducer that is attached to the surface of the structure with vacuum grease, vaseline, or other couplants to provide an air-free path for the high frequency waves to reach the active element of the transducer. The transducer used in the majority of the tests has a resonant frequency of approximately 150 kHz. When the AE waves strike the transducer, they set it "ringing" at its resonant frequency. The use of a resonant transducer increases the sensitivity of detecting the AE, events. Since the frequency contents of the waves are very broad band, they will activate the resonant frequency of any transducer having a resonant frequency between 20 kHz and 1 MHz. Most AE data is taken in the 100 to 500 kHz frequency range, where the data is low enough in frequency that attenuation effects are minimal, and high enough in frequency so that low frequency air borne noise is eliminated.
A majority of the practical AE tests are conducted on structures made from plates or plate-like components. Recent research has shown that out-of-plane (OOP) Al sources produce strong flexural wave components in a plate, with weak extensional components, while in-plane (IP) AE sources produce strong extensional waves in a plate with weak displacement components normal to the plate surface.
For many years, researchers and field test engineers employing acoustic emission techniques have used the breaking of a pencil lead on the surface of a structure or specimen to simulate the type of AE signal present when a crack propagates or when fibers break in composite structures. Because this is an OOP source, most of the energy goes into the flexural wave which is inherently low frequency, and only a small portion of the energy is carried by an extensional wave.
Michael R. Gorman, in his paper entitled "Plate Wave Acoustic Emission," Journal of Acoustic Society of America, 90(1), July 1991, used broad band sensors to detect both types of waves. By mounting the transducer on the surface of an aluminum plate, extensional waves were simulated by breaking pencil leads on the edge of the plate, and flexural waves were simulated by breaking the pencil lead on the surface of the plate. Further work by Gorman and Prosser, "AE Source Orientation by Plate Wave Analysis," Journal of Acoustic Emission, Vol. 9, No. 4 (1990), consisted of machining slots at different angles in a plate to observe the response when pencil leads are broken at an angle, which is measured from the plane of the plate. As expected, it was found that for 0 degrees, the highest signal amplitudes occurred for extensional waves, and for 90 degrees, the highest signal amplitudes occurred for flexural waves. A mixture of both waves was found for intermediate angles.
The broad band transducer used by Gorman and Prosser is problematic for a number of reasons. First, it was designed for ultrasonic testing with a resonant frequency of 3.5 MHz, and was presumed to have a flat frequency response from a few kHz to 1 MHz. However, although the measured frequency response of similar transducers yields a fairly flat frequency response from 300 kHz to 1 MHz, it is far from flat from 10 kHz to 300 kHz. In addition, its sensitivity in the frequency range below 1 MHz is an order of magnitude lower than those of resonant transducers normally used for acoustic emission testing. Consequently, the sensor must be placed close to the source in order to obtain good results. Moreover, when IP and OOP amplitudes are compared from data obtained by the transducer mounted on the surface, there is a large difference in the peak amplitudes measured from the different sources. Further, the procedure presently employed by Gorman cannot be used for crack growth measurement. Gorman merely discloses a method to digitize all signals and to attempt through visual examination and pattern recognition software to determine the amount of IP and OOP components present to make a decision regarding whether or not a signal is primarily one or the other type. Because the overwhelming number of AE signals detected in the field are extraneous noise (OOP), Gorman's method requires an enormous amount of storage for the digitized signals.
One of the major reasons AE techniques have not been widely accepted is that the techniques will not give any quantitative information concerning the size of a crack or the amount of crack growth. Currently, AE is primarily used to locate the crack by the use of multiple channels. By measuring the time that each transducer receives the AE signal, and ascertaining the velocity of sound in the material, the source location can be calculated.
The second main reason for the lack of wide application of AE techniques to monitor structures in the field is the difficulty in separating extraneous background noise from the AE signals coming from the crack. Impacts on the field structure from wind-blown sand, particles, rain, maintenance personnel, and leaks in pressurized components all can give noise signals in the frequency band of interest. Rubbing friction between components is another source of extraneous noise that has frequency components in the frequency range of interest. Most extraneous noise sources of this type are out-of-plane (OOP) sources, and although they can have very high-frequency components in an undamped structure, most of the energy in the stress waves created by such sources in most structures constructed from plates can be found at frequencies below 100 kHz. This energy is carried by a low-frequency flexural wave in the plate. AE signals generated by crack growth, on the other hand, are in-plane (IP) sources, and most of the energy in the stress wave is carried by high-frequency extensional and shear waves.